Set the nixpkgs platform as a module option.

nix/lib.nix

@@ -8,8 +8,7 @@ let
 in
 {
   makeSystemConfig =
-    { system
-    , modules
+    { modules
     , extraSpecialArgs ? { }
     ,
     }:
@@ -34,6 +33,8 @@ in
         ] ++ modules;
       }).config;
 
+      system = config.nixpkgs.hostPlatform;
+
       returnIfNoAssertions = drv:
         let
           failedAssertions = map (x: x.message) (lib.filter (x: !x.assertion) config.assertions);

nix/modules/default.nix

@@ -9,6 +9,15 @@
   ];
 
   options = {
+
+    nixpkgs = {
+      # TODO: switch to lib.systems.parsedPlatform
+      hostPlatform = lib.mkOption {
+        type = lib.types.str;
+        example = "x86_64-linux";
+      };
+    };
+
     assertions = lib.mkOption {
       type = lib.types.listOf lib.types.unspecified;
       internal = true;

test/nix/flake.nix

@@ -1,24 +1,16 @@
 {
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    flake-utils.url = "github:numtide/flake-utils";
 
     system-manager = {
       url = "github:numtide/system-manager";
-
       inputs.nixpkgs.follows = "nixpkgs";
-      inputs.flake-utils.follows = "flake-utils";
     };
   };
 
-  outputs =
-    { flake-utils
-    , system-manager
-    , ...
-    }:
+  outputs = { system-manager, ... }:
     {
       systemConfigs.default = system-manager.lib.makeSystemConfig {
-        system = flake-utils.lib.system.x86_64-linux;
         modules = [
           ./modules
         ];

test/nix/modules/default.nix

@@ -4,6 +4,8 @@
 }:
 {
   config = {
+    nixpkgs.hostPlatform = "x86_64-linux";
+
     environment.etc = {
       foo = {
         text = ''